Nickel 63 electron capture detector

ABSTRACT

A gas ionization detector is constructed to have a stacked array of individual elements together defining a sealed cavity disposed within mating cylinder and piston-housing members. The piston is adapted to threadedly engage the hollow cylindrical housing. Disposed within the cylindrical housing is a first electrically nonconductive washer, a disclike anode electrode, a second electrically nonconductive washer, and a disc element containing the radioactive material. Mounted within a bore formed in the piston portion of the housing is a thrust bearing, a spacer washer, and a disclike cathode electrode. A tube extending through the spacer washer of the piston element communicates with the outside of the detector. An orifice in the cylindrical housing provides a second access to the outside of the detector. When the piston-housing member is inserted into the cylindrical housing and tightened, the stacked elements within the housing form an inner sealed cavity which is capable of relatively high temperature operation and facilities removal and replacement of the radioactive material.

United States Patent [72] Inventors Donald F. Taylor West Grove, Pa.; Urban James Peters, John A. Schmit, Newark, Del. [21] Appl. No. 691,673 [22] Filed Nov. 24, 1967 [45] Patented Feb. 23, 1971 [73] Assignee Hewlett-Packard Company,

Palo Alto, Calif.

[54] NICKEL 63 ELECTRON CAPTURE DETECTOR 1 Claim, 4 Drawing Figs.

[52] US. Cl 250/44, 250/43.5, 250/83.6, 324/33 [51] Int. Cl ..G01n23/12 [50] Field ol'Search 250/83.6 (FT), 43.5 (R), 44; 324/33 [56] References Cited UNITED STATES PATENTS 3,378,725 4/1968 Bochinski 250/83.6FTX 3,417,238 12/1968 I-lartmann 250/83.6FTX

Primary Examiner-Archie R. Borchelt Assistant Examiner-Morton J. Frome Attorney-Mortenson & Weigel ABSTRACT: -A gas ionization detector is constructed to have a stacked array of individual elements together defining a sealed cavity disposed within mating cylinder and piston-housing members. The piston is adapted to threadedly engage the hollow cylindrical housing. Disposed within the cylindrical housing is a first electrically nonconductive washer, a disclike anode electrode, a second electrically nonconductive washer, and a disc element containing the radioactive material. Mounted within a bore formed in the piston portion of the housing is a thrust bearing, a spacer washer, and a disclike cathode electrode. A tube extending through the spacer washer of the piston element communicates with the outside of the detector. An orifice in the cylindrical housing provides a second access to the outside of the detector. When the piston-housing member is inserted into the cylindrical housing and tightened, the stacked elements within the housing form an inner sealed cavity which is capable of relatively high temperature operation and facilitates removal and replacement of the radioactive material.

NICKEL 63 ELEQTRON CAPTURE DETECTOR This invention relates to a gas ionization detector and, more particularly, to a gas detector employing a radioactive source for ionization purposes which detector is capable of operating at relatively high temperatures.

In the field of gas chromatography, a sample of a mixture to be analyzed is entrained in a flowing gas system and passed through a column packed with a bed of separatory material which functions to selectively retard the different components of the sample mixture. These sample components therefore elute from the column spaced in time. A detector monitors the eluent from the gas chromatograph column and detects changes in its composition. These changes in composition are indicative of the different sample components as they are eluted.

One of the detectors which is of great use in gas chromatography particularly for sensing pesticides, carcinogens, or any compound with afi'mity for electrons is the now well-known electron capture detector. The heart of this detector is a small quantity of radioactive material such as tritium, nickel 63 or other radioactive isotopes which provide a constant source of radiation (nickel 63 emits beta particles).'The eluent from the gas chromatograph column is passed through this radioactive field and becomes ionized. At the same time, an electric field is established across the ionized region. This electric field acts to draw the electrons from the ionized eluent to the anode. By measuring the current flow between the anode and cathode of the electron capture detector and observing changes in its amplitude, changes in the constituency of the column eluent may be observed. With the presence of an electron-capturing compound, such as the pesticides, the measured signal decreases as the electron-capturing compound tends to combine with the free electrons. A detector of this type is extremely sensitive and is able to detect quantities as small as gram.

A particular deficiency in these detectors has been the difficulty of replacing or loading the cell during manufacture with the radioactive foil. Because of the inherent danger, such loading must take place in a hot lab and once loaded, the cell must be tightly sealed so as to prevent the escape of any radioactive material. With this limitation loading these cells has been a problem. Most cells have been difficult to load because of their elaborate construction made necessary to safely contain the radioactive material.

In addition it has been relatively difficult to construct a cell that was capable of operating at high temperatures in the range of about 300-400 C. At these high temperatures the radioactive material volotalizes and invariably tends to escape from the cell, thereby endangering persons using the cell. Materials that would normally provide adequate sealing of the cell tend to permit leaks at these higher temperatures.

it is therefore an object of this invention to obviate many of the disadvantages inherent in the prior art electron capture detectors.

Another object of this invention is to provide an improved electron capture detector which is capable of operating at relatively high temperatures in the range of 300400 C.

Still another object of this invention is to provide an improved electron capture detector which is capable of being assembled and disassembled relatively easily.

An additional object of this invention is to provide an improved electron capture cell in which the radioactive material may be relatively easily loaded in the cell.

In accordance with a preferred embodiment of this invention an electron capture detector is constructed to have a hollow cylindrical housing and a pistonlilte end cap which threadedly engages the open end of the cylindrical housing to provide a first chamber. Within the first chamber are disposed a stacked array of elements which, when placed under compression as by tightening the pistonlilte element, form an inner sealed chamber including an anode and cathode, a radioactive element disposed therebetween, and inlet and outlet ports for passing a gas between the electrodes and through the radioactive field. The stacked array of elements is constructed of materials capable of withstanding relatively high temperatures to permit high temperature operation. The radioactive elements may be removed, replaced, etc. simply by removing the pistonlike element and replacing the now exposed radioactive material.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation as well as additional objects and advantages thereof will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view of an electron capture cell constructed in accordance with this invention;

FIG. 2 is a cross-sectional view taken along the axis of the electron capture cell illustrated in FIG. 1;

FIG. 3 is a sectional view of the electron capture cell of this invention taken along the section line 3-3 of FIG. 2; and

FIG. 4 is a section view of the electron capture cell of this invention taken along the section lines M of FIG. 2.

The electron capture detector of this invention is constructed to have a housing comprising a cylindrical portion 10 and a pistonlike end cap 12. The cylindrical portion of the housing 10 is hollow and has one end 12 open with internal threads 14 adapted to screwingly engage the external threads 13 of the pistonlike end cap 12. The other end of the cylindrical housing 10 has an end face which contains a central axial orifice 18 into which a suitable conduit 20 may be introducedas desired. The conduit may be of conventional stainless steel tubing and may be welded as denoted by'the head 22 to provide an appropriate seal. At some radial point in the end portion of the cylindrical housing 10, a second orifice 24 is formed to permit, as will be described, the introduction of a wire 52 leading to the anode of the electron capture cell.

The cylindrical housing '10 and the pistonlike end cap 12 may be formed from suitable metal bar stock such as stainless steel or aluminum and machined into the desired configurations. The orifice 18 may include the outwardly extending pip 24, if desired, to provide better support for the tube 20 but is not necessary.

When thus assembled the pistonlike end cap 12 and cylindrical housing 10 form a cavity or first chamber 30 in which is disposed a stacked array of elements which, when placed under compression as by screwing the pistonlike end cap 12 into the cylindrical housing 10 so as to place the stacked array under compression, forms a second chamber 32. The eluent gases from a gas chromatograph, directed by the conduit 20, pass through the second chamber 32 for detection. As seen from left to right in the drawing of FIG. 2, this stacked array of elements includes a metal O-ring 40 which is placed in an annular coaxial groove 42 formed in the interior end face of the cylindrical housing 10, Seated against this end face and against the O-ring 40, which may be formed of stainless steel, is a first washer element 44. The washer element 44 is formed of a chemically inert, electrically nonconductive material which is capable of withstanding relatively high temperatures such as boron nitride or alumina. Other suitable ceramic materials may be as desired. The first washer 44 has a diameter slightly less than the inside diameter of the hollow cylindrical housing 10 and on its right-hand face has a raised disclike portion 46 of lesser diameter. As the name implies, the washer 44 has a central orifice 48 of approximately the same diameter of the tubing 20.

At a radial point along the periphery of the washer 44 an axial orifice 50 is formed as by drilling to permit the wire 52 passing through the orifice 24 to extend into the interior of the housing and connect eventually to the anode element 54. The anode element 54 is placed adjacent the washer 44. Conventional bead-type insulation 56 may be placed about the wire 52 to insulate it from contact with the housing 10 and from any other exterior parts. The head insulation must be capable of withstanding high temperatures. Suitable beads are available commercially made of steatite ceramic.

The anode 54 is a disclike element formed of a metal such as stainless steel and has a peripheral annular flanged portion which extends in either direction along the axis of the housing. A second O-ring 6G is placed within a recess formed by the flange of the anode 54- and engages the raised disc portion 46 of the washer M. The flanged portion of the anode 54 has a stepped recess portion 62 which is adapted to fit over the periphery of the raised portion 46 of the washer 44 so as to maintain the proper coaxial alignment of the anode 54 within the cylindrical housing It). A plurality of symmetrically disposed holes 64 (H6. 3) are formed in the anode element 54 to permit gas from the conduit 20 to flow through the anode into the ionization chamber itself as will be described. The right-hand face (in the drawing) of the anode 54 has retained therein by the peripheral flange of the anode element 54 a third O-ring 66 which engages a second washer 70.

The second washer 70 may be formed of the same material as the first washer 44 and has a diameter slightly less than the internal diameter of the cylindrical housing so as to permit its ready insertion without jamming into the housing. This diameter should not be so loose so as to permit appreciable free play. The right-hand face of the second washer 70 has a peripheral flanged portion extending axially to the right in the drawing so as to provide a seat for a disclike member 74. The disclike member 74 is formed preferably from 24 karat or better gold and has plated thereon nickel 63 or other suitable radioactive material preferably one capable of high temperature operation. A plurality of holes (FIG. 4) and a central orifice 76 are formed in the disclike member 74 to permit the flow of gas there through. The second washer 70 has a relatively large diameter and provides the major volume of the ionization chamber.

A thrust bearing 78 is disposed within a counter bore 80 in the pistonlike end cap 12. The pistonlike end cap 12 also has a central bore 82 which extends axially through the entire end cap. Next, moving to the left in the drawing is a washer 77 which engages the ball hearing within the thrust bearing 78 and permits the force, as the end cap 12 is tightened within the cylindrical housing 10, to be distributed equally and axially against the stacked elements as will be seen.

Next in the stacked array is a third spacer washer 84 which must be an electrically nonconductive material, but need not this case be chemically inert since it does not form an exposed part of the ionization chamber. The only requirement for the third washer 84 is that it be capable of withstanding high temperatures. A ceramic suitable for this purpose is alumina which is formed from pressed aluminum oxide which is machined in the green state and fired. The spacer washer 84 has an outer diameter slightly less than the inner diameter of the counter bore 88 within the end cap 12 to permitit to be easily introduced therein but without appreciable free play. To complete the assembly, the cathode 86 is positioned within the spacer washer 84. The cathode is in the form of a disclike element as having a peripheral axially extending disclike flange 8% which engages the peripheral portion of the disclike member 74 containing the radioactive material. The righthand face (in the drawing) of the cathode 86 has an axially extended tubular portion 90 which serves to coaxially position the cathode 86 within the third washer 84 and permits the introduction of an exit conduit 92 which may be welded as noted at 94 to the tubular portion 90 of the cathode 86. The exit conduit 92, as was the case for the inlet conduit 20, may be of conventional stainless steel tubing of appropriate diameter.

To assemble the cell, the several elements are merely inserted in the order described in their respective mounting assembiies, i.e., the cylindrical housing and the end cap 12. Next the disclike member 74 is plated with a radioactive material. This operation is typically performed in a hot lab where appropriate precautions for the handling of radioactive materials may be taken. The radioactive member 74 is now inserted into the cylindrical housing and seated in the seat in the face of washer 70 and the end cap 12 inserted into the cylindrical housing 110. The end ca 52 is screwed until the several elements become compresse A typical compression torque is used in practice to permit the'desired seal is l08 to inch-pounds for a cylindrical housing approximately 1 inch in exterior diameter. Under compression, a tightly sealed cavity 32 is formed which is completely isolated from the exterior housing 10 and the end cap 12 by the first and second washers 44 and 70, respectively, the anode the disc 74, and the cathode 86.

During usage of the cell, eluent from the gaschromatograph flows inwardly through the inlet conduit 26) around and through the holes 64 in the anode 54, through the cavity formed within the second washer 70 at which time the gases are exposed simultaneously to the radioactive radiation and to the electric field established between the anode 5d and the cathode 86 (and the radioactive member 74 which is also electrically conductive). The gases continue their flow out through the holes 76 in the radioactive member 74, through the central orifice of the cathode 86, and thence out through the exit conduit 92. The cell is relatively leak free and if any part becomes damaged or contaminated, it can be readily replaced.

The description of the detailed operation of the electron capture cell will be omitted since it is well known. Suffice it to say, that by the use of the ceramic elements capable of withstanding high temperatures to form the chamber along with the O-ring seals and the anode and cathode elements, the chamber 32 is tightly sealed, leaving a small exterior chamber 30 within the cylindrical housing '10 which is of no consequence. Thus formed, the electron capture cell is relatively easy to load. All that need be done to effect the loading is to remove the end cap 12, insert the radioactive member 74, engage the end cap 12 within the housing 10, tighten and the cell is loaded.

it is obvious that many embodiments of this inventive concept may be made, and that many modifications may be made in the embodiments hereinbefore described. Therefore, it is understood that all descriptive matter herein is to be interpreted merely as illustrative, exemplary, and not in a limited sense. It is intended that the various modifications which might readily suggest themselves to those skilled in the art be covered by the following claims, as far as the prior art permits.

We claim:

1. A gas ionization detector comprising:

a hollow cylindrical housing member having an axis, a

closed end and an open end;

a pistonlike element adapted to threadedly engage the open end of said housing member thereby defining a first cavi- W; said element having a central bore and counterbore coaxial with the axis when engaged with the housing member; said closed end of said housing member having a coaxial bore; a disclike anode element; a disclike cathode element adapted to have disposed thereon the radioactive material nickel 63;

each of said anode and cathode elements being electrically conductive and having orifices to permit gas flow therethrough;

a plurality of chemically inert, electrically nonconductive spacer washers;

each of said anode and cathode elements being disposed in a stacked array within said first cavity along said axis separated from each other and from said housing member and said pistonlike element by one of said washers, thereby defining a second cavity within said first cavity; and

means to apply a potential between said anode and cathode elements. 

